These books are project-based and are best suited for beginners to intermediates who have a cursory familiarity with electronics.
1. Make: Electronics: Learn by Discovery
2. Make It, Wear It: Wearable Electronics for Makers, Crafters, and Cosplayers
4. Make: Wearable Electronics: Design, prototype, and wear your own interactive garments
Traditional, classroom-based, thorough instruction:
Self-paced modules:
Learn-c.org is a free online portal for learning C, the basis of Arduino, C, C++, and other programming languages
Adafruit Learn Arduino blog posts
Reference materials: